Chemical Energetics
Chapter 9, presented by Coco Chan
TABLE OF
CONTENTS
Key OBJECTIVES
Be able to describe what is meant by exothermic and
endothermic reactions
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•
•
•
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Be able to interpret energy level diagrams
showing exothermic and endothermic reactions
Describe the release of heat energy by burning
fuels
Know that hydrogen is used as a fuel
Know that radioactive isotopes, such as 235𝑈, can
be used as a source of energy
Know that bond breaking is an endothermic
process and bond formation is an exothermic
process
Be able to draw and label energy level diagrams
for exothermic and endothermic reactions using
data provided
Be able to calculate the energy change in a
reaction using bond energies
•
Energy Change
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There is always an energy change during a
chemical reaction.
Also known as enthalpy change ΔH
Chemical reaction: breaking and reforming
bonds
Exothermic reaction: give out energy
Endothermic reaction: take in energy
What happens
in a chemical
reaction?
Chemical reactions are like building lego blocks!
Car  House
1. Take apart the block
2. reassemble in a different way
What happens
in a chemical
reaction?
Step 1
Bond-breaking
Step 2
Bond-forming
Energy changes during a chemical reaction:
Step 1: during bond-breaking, energy is needed (heat is absorbed) - endothermic
Step 2: during bond-forming, energy is given out (heat is released) - exothermic
In an exothermic
reaction:
Heat energy given out to the surrounding, so the
temperature we measured increases.
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Combustion of fuels (including respiration)
Many oxidation reactions
Neutralisations
In an endothermic
reaction:
Heat energy is taken from the surroundings, so the
temperature decreases
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Thermal decomposition
photosynthesis
Energy Level
Diagram
Diagrams showing the enthalpy change during an exothermic
and endothermic reaction
Test yourself
1. A student adds a small piece of sodium into a
beaker of water. The temperature of the water
increases from 21︒C to 39︒C.
Which type of reaction (exothermic or
endothermic) has occurred and why?
2. Two steps are needed, to go from reactants to
products in a chemical reaction.
What are they?
Drawing energy
profile diagram
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Draw the horizontal and vertical axes. Label the axes.
Draw two bars (different heights):
○
Endothermic – reactant < product
Bond breaking – energy is taken in, arrow always points
upward
Bond forming – energy is given out, arrow always points
downward
○
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●
exothermic – reactant > product
Bond breaking and
bond forming
We know that when a reaction takes place, bonds are
broken, and new bonds formed.
What bonds are broken and what bonds are formed
during the following reaction?
𝐻2 + 𝐶𝑙2
2 𝐻𝐶𝑙
Activation energy is the energy required to break all the
necessary bonds in the reactants.
The stronger the bonds are, the more energy is needed
to break them.
Calculating the energy
changes in reactions
The molar enthalpy change for reactions can be calculated:
𝛥𝐻 = energy taken in – energy given out
Consider reaction between hydrogen and chlorine again:
1 H-H bond is broken:
1 Cl-Cl bond is broken:
Total energy taken in =
2 H-Cl bonds are made:
Total energy given out =
Overall, there is less energy taken in than given out.
Therefore, overall energy is being released
– this reaction is exothermic.
3. Identify the type of reaction from the following
energy level diagram.
4. Hydrogen Bromide decomposes to form hydrogen
and bromine.
2𝐻𝐵𝑟
𝐻2 + 𝐵𝑟2
Calculate the enthalpy change and
identify the type of reaction.
Going over some important
terms
Exothermic
reaction
●
Energy given out to the
surroundings
●
Surroundings become hot
●
Bond making - exothermic
Activation energy
Activation energy is the
energy required to break all
the necessary bonds in the
reactants.
Endothermic
reaction
● Energy is taken from the
surroundings
● Surroundings become cold
● Bond breaking -
condothermic
Activation
Energy
Ea 活化能
Energy needed to break all the
bonds in the reactants –
minimum energy needed to
start off a reaction.
If Ea is low, not much energy is needed. For example, mixing water with
calcium oxide, the reaction can start spontaneously.
•
If Ea is high, some reaction requires extra energy to start off. For example,
the reaction between iron and sulfur, requires heat. Then the energy given out
can support the reaction to break further bonds.
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But for endothermic reactions, for example the decomposition of calcium
carbonate, you must continue heating until the reaction is complete.
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Energy from
fuel
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A fuel is any substance we use to provide
energy.
E.g. fossil fuel, hydrogen, ethanol
Exothermic or endothermic?
What makes a
good fuel?
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The amount of energy is given out
Availability
Be a liquid at room temperature
Affordability
Environmental friendly
Hydrogen as
a fuel
Hydrogen is used in rocket engines and in fuel cells to
power some cars
Advantages:
• It releases more energy per kilogram than any other
fuel (except for nuclear fuels)
• It does not pollute as it only produces water on
combustion, no other product is formed
Disadvantages:
• Expensive to produce and requires energy for the
production process
• Difficult and dangerous to store and move around
(usually stored as liquid hydrogen in highly
pressurised containers)
Hydrogen fuel cell
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•
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A fuel cell is an electrochemical cell in
which a fuel donates electrons at one
electrode and oxygen gains electrons at
the other electrode
These cells are becoming more common
in the automotive industry to replace
petrol or diesel engines
H2 and O2 are pumped through two
porous electrodes where the halfreactions occur
The following reaction occurs at the
anode:
Radioactive
isotopes as
fuel
Uranium-235 undergoes decay and gives off heat energy
which nuclear power stations harness
The heat it produces is used to heat water to steam, which in
turn is used to power turbines to generate electricity
Nuclear fuel energy is clean as it does not produce pollutants
such as CO2 or oxides of nitrogen or sulfur
But nuclear power plants are expensive to build and maintain
as well as being potentially dangerous in the event of an
accident as radioactive materials may be released
SCHEDULE
APRIL
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TABLE
TEST 1
Describe here your metrics 1
Describe here your metrics 2
Describe here your metrics 3
Describe here your metrics 4
Describe here your metrics 5
TEST 2
TEST 3
TEST 4
THANKS
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